1. Field of the Invention
The present invention relates to a printer system and a printer in which image data having pseudo gradation generated by using a dither pattern are compressed by a computer and transmitted so that a printer can receive and expand the compressed data for printing out.
2. Description of the Prior Art
In general, there is a print system that includes a computer and printers. For such print systems, there is a system in which image processing such as pseudo gradation for image data to be printed is performed on the computer side, and a printer side receives the image data from the computer and outputs the image data without a special process for printing on a sheet of paper.
Conventionally, there is a printer that is used for this system, which is called a raster printer. The raster printer can be realized at low cost by using a small scale of hardware because a special image processing is not performed on the printer side. However, it has a disadvantage that data quantity to be transmitted becomes large because it is necessary to transmit image data that can be printed on a sheet of paper as it is to the printer. In addition, if the image data includes character data, a resolution should be raised for a clear print of characters. As a result, the quantity of data to be transmitted to the printer becomes larger. Therefore, the transmission of image data from a computer to a printer requires a lot of time.
In order to solve this problem, it is necessary to compress the image data on the computer side. However, if a compression process is complicated, the compression process itself may require a lot of time, and the printer side may also require a lot of time for an expansion process so that a scale of hardware and its cost increase.
Therefore, a compression technique is desired in which the compression process is light, a small scale of hardware is sufficient for expanding data and a high compression rate can be realized in order to realize a print system in which an inexpensive printer can be used and that enables a high speed process as a whole.
In order to print image data of multi gradation by a printer, it is necessary to realize the pseudo gradation of the image data in many cases. Conventionally, a dithering method is known well as a method for realizing the pseudo gradation. The dithering method uses a dither pattern and compares each of threshold values of the dither pattern with a value of each pixel in the image data for producing binary values.
There is a compression method that notes a dither pattern as disclosed in Japanese unexamined patent publication No. 6-152986. According to this method, the binarized image data are divided into blocks having a predetermined size. A reference pattern that corresponds to the number of dots included in any image block is compared with the image block. If the image block is identical to the reference pattern, a code indicating the reference pattern is assigned. Otherwise, a result of the comparison is coded.
In addition, as a compression technique for the binarized image data, there is a JBIG format that is approved to be adopted by ITU (International Telecommunications Union)-T recommendation.
However, according to the former method described in Japanese unexamined patent publication No. 6-152986 mentioned above, data quantity to be sent increases by the quantity of codes of the reference pattern because the reference pattern is coded when the designated image block is identical to the reference pattern.
On the contrary, according to the latter method of JBIG, the compression process is relatively light so that a computer can perform the compression process at a high speed. However, a data compression ratio of the JBIG method is not so high as other method such as an LZS method or an LZW method. Especially, the data compression ratio has tendency to decrease if half tone data increases.